In manufacturing said composite pieces, it is known that fiber reinforcement is made which is then imprisoned in a settable matrix. Fibers suitable for use in making such fiber reinforcement are, for example, carbon fibers, boron fibers, glass fibers, aramide fibers, etc. Such fibers are designated below as "technical fibers ".
It is also known that when making such fiber reinforcement, stitching methods are often implemented using such technical fibers. For example, such stitching may be used:
for assembling together individual fabric pieces to make up the desired reinforcement; PA1 for bending layers of superposed fiber material together to make up a fabric piece, from which reinforcement or a reinforcing element is subsequently made; PA1 for inserting fibers in a desired direction in a piece of fabric. PA1 said needle thread is constituted by a flexible thread that is strong in traction; PA1 said shuttle thread is constituted by said thread of technical fiber; and PA1 the tension of the shuttle thread and the tension of the needle thread are respectively adjusted to a level that is sufficiently low and a level that is sufficiently high to ensure that after said stitch has been made, said shuttle thread projects outside said piece of fabric from the first face side thereof, and said needle thread lies completely outside said piece of fabric, likewise on said first face side thereof.
Stitching tests performed on such pieces of fabric using continuous threads made up of technical fibers and stitched by means of conventional sewing machines have been disappointing or completely impossible. Given the nature of technical fiber threads, they are damaged by the needle used for stitching them in said pieces of fabric, and this occurs to such an extent that they break frequently, thereby stopping the current stitching operation. Furthermore, the thicker the piece of fabric, the greater the frequency at which the stitching threads break, thereby making it impossible to stitch thick pieces of fabric.
In any event, even when a technical fiber thread does not break during stitching, it may break after stitching since a fragile zone is created at each stitch where the needle thread engages the shuttle thread.
Thus, to be able to stitch such a piece of fabric using a thread of technical fiber, it has been necessary to make stitching devices that are complex and/or that include a special support for the piece of fabric to be stitched, e.g. as described in documents U.S. Pat. Nos. 2,283,802 and 3,322,868.
An object of the present invention is to remedy this drawback and to allow pieces of fabric made of technical fibers to be stitched by means of threads likewise made of technical fibers, by using conventional sewing machines, and even when said pieces of fabric are thick.